The present invention pertains to a method and apparatus for removing added pulses from a video signal to render copiable the video signal. More particularly, the present invention pertains to such a method and process for removing pseudo-sync pulses which have been added to a video signal, or AGC (positive) pulses added to a video signal for the purpose of inhibiting copying of the video signal. The present method and apparatus are also useful with various combinations of the added pulses stated above.
In a copending application by the present inventor entitled METHOD AND APPARATUS FOR PROCESSING A VIDEO SIGNAL SO AS TO INHIBIT THE MAKING OF ACCEPTABLE VIDEO TAPE RECORDINGS, Ser. No. 554,697, filed Nov. 23, 1983, and now abandoned, there is disclosed a method and apparatus for processing a video signal in a manner so as to inhibit the making of acceptable videotape recordings thereof. However, the processing of the video signals there disclosed is such that a conventional television receiver produces a normal color picture from the modified signal.
In another copending application by the present invention with the same title as above, Ser. No. 724,006, filed Apr. 7, 1985, and now U.S. Pat. No. 4,631,603 the same general objects and purposes of the Ser. No. 554,697 invention are also achieved. In other words, both of these inventions disclose how a video signal can be modified so that a television set produces normal video pictures from the modified signal, but a videotape recording made from the modified signal produces generally unacceptable pictures.
Both of the foregoing "inhibit" inventions rely on the addition of high level positive pulses (hereinafter referred to as AGC pulses) to the video signal, following the trailing edges of some percentage of the sync pulses. This pulse addition occurs during the back porch region of the video signal, i.e. within the blanking interval, in accordance with the first of those inventions. These added pulses cause the video signal level automatic gain control system in a videotape recorder to make an erroneous assessment of video signal level, thereby leading to an unacceptable or bad recording of the video signal.
According to the several inventions above, the sync pulses referred to can be either the normal sync pulses of the video signal (including both equalizing and broad pulses) or the pseudo-sync pulses added to the video signal in accordance with the second of those invention. The pseudo-sync signals, which extend from the blanking signal level of the video signal to the normal sync tip level, are added to the video signal during some of the lines of the vertical blanking interval. Alternatively, blanking interval lines may be added immediately preceding and/or following this normal vertical blanking interval. These added lines would then be especially blanked for the "inhibit" feature purpose. According to the method of these inventions, each of the pseudo-sync pulses will be followed respectively by an AGC pulse (i.e. positive pulse), whereas only a fraction of the normal sync pulses in a video signal would be expected to be followed by an AGC pulse.